The art of burning solid fuel is receiving increased attention during the lengthening energy crisis. One of the developing techniques for the consumption of solid fuel includes reducing the solid fuel to particle size of about 1/4" in diameter and combusting a bed of this material while the bed is fluidized.
The primary solid fuel is coal. This fossil fuel is granulated and supplied continuously to the combusting bed which is physically supported above a perforated structure up through which combustion air is forced. Fuel pipes are mounted to extend up through the perforated support and terminate within the expanded bed. A diverter structure is provided above the upper end of each of the fuel pipes to divert the fresh supply of fuel particles horizontally into the combusting bed. The fuel is transported up the fuel pipes by entrainment in air which moves the particles at a velocity in the order of thousands of feet per minute. Diverted horizontally into the combusting bed, the granulated solid fuel is provided with a residence time in the order of a few seconds before being entrained in the vaporized products of combustion traveling upward from the bed at a superficial velocity in the order of 7 feet per second.
The temperature of the combusting bed will vary, depending upon the type of coal available. The temperature of the combusting bed is expected to be generally in the order of 1550 F. With the freshly transported granulated coal passing through the combusting bed with only a residence time of a few seconds, complete combustion is not being obtained. The more volatile substances of the granulated coal ignite at about 800 F., but the carbon of the coal must reach the temperature of about 1300 F. The result, presently, is that a significant amount of freshly supplied fossil fuel escapes from the bed and must be recycled.
A cyclone, or centrifuge structure, presently receives the products of combustion discharging from the fluidized bed. The fuel particles not reduced to ash are collected and recycled to the fuel pipes. It is, of course, desired to reduce all of the solid fuel efficiently to ash and any limestone which has been added and is now spent which can be drawn from the bed through strategically placed drain pipes extending down through the perforated support, or carried downstream with the products of combustion for bag-house collection.
Thus, the present art needs to provide a greater residence time for the fresh granulated solid fuel discharged through its fluidized bed to more efficiently reduce the particles in combustion. The solid particles discharged from the upper end of the fuel pipe must be diverted into a path sufficiently elongated to provide the required residence time within the hottest portion of the combusting bed. Those portions of the bed adjacent the discharge of the fuel pipes are heated by the combustion of the volatiles of the freshly supplied fuel, and it is in these combustion zones that the freshly supplied fuel particles must be provided the residence time necessary for complete combustion.